Catheter delivery systems usually comprise a sheath at whose tip is contained the device to be implanted. The device is usually highly compressed, particularly in diameter, and the device is released by drawing the sheath back, allowing the device to emerge from its tip. All designs feature a structure which provides a reaction force for the device so that it maintains its position while the sheath is being withdrawn.
A simple example of this type of delivery system is provided with the Talent™ stent-graft manufactured by Medtronic, Inc. The system suffers two principal draw-backs. The first is that the highly compressed implant pushes against the wall of the sheath with such force that withdrawing the sheath also requires a considerable force. A second (allied) problem is that providing the necessary force to deploy the stent-graft can be difficult for the medical practitioner to achieve with accuracy or control, allowing the implant to be deployed too quickly and, potentially, displaced from its intended landing site.
An improved system has been incorporated into the delivery system supplied with the AneuRx™ stent-graft, also manufactured by Medtronic. In this system, a barrel-shaped component of the handle of the delivery system is rotated and acts on a lead-screw to withdraw the sheath with both mechanical advantage and precision.
Additional complexity is added in this system because the sheath stretches significantly while it is being withdrawn. When the medical practitioner's hand is being repositioned on the barrel of the handle, the barrel is free to rotate and is prone to spin back as the sheath springs back to its original length. The AneuRx™ system therefore employs a conventional ratchet mechanism on the rotating barrel to prevent this elastic recoil.
Introduction of the ratchet incurs a further complexity. When the delivery catheter is being withdrawn from the patient, it is highly desirable that the sheath be slid back into its original position to cover the remaining internal components of the delivery system so that they do not inflict any trauma onto the viscera of the patient. For this purpose, the AneuRx system employs a separate control to override the ratchet and allow the barrel to be rotated in the opposite sense to allow the sheath to be returned to its starting position.